Currently, there are a multitude of wireless systems in place, including, inter alia, four varieties of Global System for Mobile Communications (GSM)—GSM 850, 900 GSM, 1800 GSM, 1900 GSM, as well as third generation (3G) systems and emerging fourth generation (4G) systems. BLUETOOTH® and wireless Local Are Network (LAN) capability is also being implemented in mobile phones. Users are demanding more and more functionality, and many wireless engineers are discovering that they need bigger antennas but cannot increase the sizes of handsets.
As a side effect of the popularly recognized Moore's Law for semiconductors, customers and handset suppliers expect consumer technology to keep shrinking in size and increasing in functionality, without regard to the constraints of physics. For many applications, there are fundamental size limitations of antennas that have been reached with today's technology. The antenna, unlike other components inside a handset, sometimes cannot keep decreasing in size. Before the existence of cellular systems, a scientist postulated the physical law responsible for governing antenna size, and the law is now known as “Wheeler's Theorem.” In short, Wheeler's Theorem states that for a given resonant frequency and radiation efficiency, the total bandwidth of the system is directly proportional to the size of the antenna. Further, as resonant frequency increases, antenna size usually decreases, and as efficiency increases, antenna size usually increases. Thus, changes to efficiency, bandwidth, or frequency often require changes to antenna size, and changes to frequency, efficiency, or size, often affect bandwidth. This generally represents the physical constraints facing engineers as they design antennas systems for consumer and other devices.
The implications of Wheeler's Theorem for the continued expansion of wireless systems are contrary to consumer expectations regarding bandwidth and size. Currently, antenna sizes required for tri-band GSM are 5.5 cubic centimeters (for internal antennas with a ground plane) and 2.5 cubic centimeters (for antennas without a ground plane directly underneath). The space required by antennas in handsets is currently between 5 to 20% of the total space. Generally, either antennas will become much larger to accommodate additional bandwidth, or antenna performance will decrease to accommodate smaller applications. Using what is known about current systems, it is believed that if required bandwidth doubles and performance stays the same, handset size will accordingly increase by up to 20%.
One method of balancing performance and size is to keep the bandwidth approximately constant while using circuitry to adjust the resonance properties of an active antenna system. Whereas most antennas are passive antennas with up to two connections (feed and ground) to the motherboard/Printed Circuit Board (PCB) and no additional power requirements, an active antenna uses a switching circuit to physically control parts of the antenna.
Engineers use active antenna systems to decrease antenna size while giving the appearance of attaining performance gains. The active antenna system uses a switching element to re-configure the driven antenna elements therein, changing the resonant frequency and maintaining similar efficiency and bandwidth performance for each frequency. Each setting of the antenna acts as a separate antenna for purposes of Wheeler's Theorem; thus, using an active antenna system can seem, in some respects, like receiving several antennas for the physical cost of one. Using this technique, an engineer can design an antenna system that has acceptable performance for multiple wireless networks without an increase in size. Unfortunately, these active antennas are usually very complex and very difficult to design. In addition, most of the active antenna solutions rely on a technology that has yet to be fully commercialized-low power and low-profile Radio Frequency (RF) Micro Electromagnetic (MEM) switches.
FIGS. 1-4 depict various active antenna system designs. FIG. 1 is an illustration of a switched matching circuit active antenna system 100. This system, used, e.g., in the NOKIA® 8810 handset (c. 1998), employs diode 101 to switch additional matching component 102 between antenna element 103 and RF Module 104. This can be suitable for changing the frequency resonance for a single band antenna, but is not suitable for multi-band antennas This is because a matching circuit is usually tuned for a single frequency band, and changing a single matching circuit will usually only shift the resonance by 2-5%, which is generally not enough to switch an entire frequency band for multi-band antenna applications.
FIG. 2 is an illustration of switched feed active antenna system 200. By switching between feed locations 201 and 202, it is possible to shift the resonant frequency properties of antenna element 203. This technique, however, includes on-board, high-power RF switching element 204, and it can be very difficult to avoid intrinsic losses from the RF switching element. Further, it can be difficult to independently control the resonance properties of two or more frequency bands since both resonances are dependent on the feed placement.
FIG. 3 is an illustration of switched ground active antenna system 300. By switching between ground locations 301 and 302, it is possible to shift the resonant frequency properties of antenna element 203. This technique is similar to the switched feed technique of FIG. 2, but it does not require a high-power RF switching element. However, it can be difficult to independently control the resonance properties of two or more frequency bands since both resonances are dependent on the ground placement.
FIG. 4 is an illustration of reconfigurable antenna system 400. First introduced in antenna array systems, reconfigurable antennas can be employed in patch antenna arrays. A reconfigurable patch array is shown as system 400. A set of patch antenna elements 401-404, connected by a series of RF switches 405-407 can be turned “on” or “off,” rendering them electrically invisible and effectively reconfiguring the physical geometry of the antenna system as a whole.
Reconfigurable systems, such as system 400, can become quite complex since RF switching components 405-407 often require a DC ground connection. Since such antennas usually cannot tolerate a DC ground at switching element locations, an additional microstrip line can be used to isolate the DC ground from each patch antenna element 401-404. The isolating microstrip line usually only works for a particular frequency; thus a multi-band antenna will usually require multiple isolators or a single, but complex, isolator. In addition, since the surface current on each of patch antenna elements 401-404 passes through a respective switching element 405-407, antenna performance often decreases due to the Ohmic losses in the switching element. One technique to avoid Ohmic losses is to use multiple switches per antenna element; however this increases total system cost and complexity.
In the prior art, there is no active antenna technology available that can provide performance at multiple frequency bands with a minimum of complexity. Consequently, there is no technology currently available that can provide switching for multiple band antennas at a size and a price that is desirable for wireless device consumers.